Hydrocarbons such as oil or gas are extracted from underground reservoirs through a well bored into the rock formation by means of a drill bit connected to a drill string or coil tubing. Rock cuttings are removed from near the bit by means of a drilling fluid or mud pumped down the pipe or coil tubing, which mud returns the cuttings to the surface via the annulus between drill string and the formation. In order to avoid wellbore instability the pressure of the mud at any depth should exceed the pressure of liquid in the pore space of the formation.
Commonly the mud is densified by the addition of a finely divided weighting agent, typically barite but often other minerals, the weighting agent being of density exceeding the unweighted mud density, and added in quantity judged to achieve the proper mud density. The lower limit of the particle size of the weighting agent is selected such that it does not affect the mud's rheology, neither damage the permeability of the formation in a reservoir section. An upper limit of the particle size is given by the mesh size of the shale shaker screens used to remove cuttings from the circulated mud. In practice the size lies in the 10 micron to 100 micron range. Being of this size the particles are not suspended by Brownian motion and so are prone to sediment under the influence of gravity; this process is termed "sag", or "barite sag".
A sag monitor as described herein measures the sedimentation of weight agents during downhole operations where the mud is neither circulated nor in a gelled state, e.g. during tripping, logging, or placing casing.
A device with similarity to that described herein was presented by Jefferson (ASME 91-PET-3, ASME Energy Sources Technol. Conf. & Exhibition, New Orleans, USA, Jan. 20-24, 1991). In his method the mud is sheared between the outside diameter of the rotor of Fann 35 Oilfield Rheometer, and the inside diameter of a Fann heat cup with a flat bottom. After shearing for a specified time, samples of mud are extracted from the top and bottom of the mud sample in the heat cup using a syringe inserted through the annulus at the top of the device. The density of the extracted samples was measured with either a pocket mud balance, or by weighing a known volume of extracted mud on a balance.
Among the disadvantages found in the known apparatus is a non-reproducible sampling technique. Furthermore, the apparatus tends to underestimate the sag. Also, the densest mud collects in a vertically heterogeneous layer at the base of the heat cup, and is not precisely sampled by the needle inserted from the top. Similarly there is a vertical gradient of mud density in the heat cup, leading to similar errors of density in the mud taken from the top of the heat cup.
The present invention addresses the above deficiencies in Jefferson's method, and further includes a method of real-time measurement of the sedimentation process.